The lesswrong quote seems to be misleading. It links to an article that says the vaccine may prevent / reduce transmission but that it hasn't been proven yet. Did I miss something?
The British Medical Journal has an article that makes the point, including quotes from Tal Zaks of Moderna, that the studies aren't designed to be statistically significant indicators of disrupting transmission, preventing hospitalisation, or even preventing death:
So, from what I can tell. It isn't proven to disrupt transmission or reduce hospitalisation/death. It would require much larger or longer studies, which they have opted not to do.
I think lesswrong is wrong here. I looked at the document that lesswrong links, and I couldn't find any statement about effectiveness of the vaccine in preventing infection after two doses.
https://www.bmj.com/content/371/bmj.m4037
So, from what I can tell. It isn't proven to disrupt transmission or reduce hospitalisation/death. It would require much larger or longer studies, which they have opted not to do.